Ceramic regenerator seals

ABSTRACT

A monolithic seal member made of zinc oxide is positioned in the housing of a gas turbine engine where the seal member rubs against a rotating ceramic regenerator. The seal member has excellent friction and thermal shock characteristics that make it uniquely suited for use in the gas turbine engine. Adding small amounts of calcium fluoride to the seal member reduces the coefficient of friction without changing significantly the thermal shock characteristics. The zinc oxide also can be applied as a coating to a ceramic substrate.

United States Patent Cassidy 1 Mar. 7, 1972 CERAMIC REGENERATOR SEALSDonald J. Cassidy, Plymouth, Mich.

Assignee: Fond Motor Company, Dearbom, Mich.

Filed: July 9, 1970 Appl. No.: 53,616

Inventor:

US. Cl. ..277/96, 277/235 A, 237/DIG. 6, 117/129 Int. Cl ..Fl6j 15/54Field of Search .....277/235 A, 96, DIG. 6; 165/9; 117/1052, 129, 100 BReferences Clted UNITED STATES PATENTS Sawchuk 165/9 3,368,612 2/1968Brummet ..277/96X 3,120,919 2/1964 Gardiner ..277/96X PrimaryExaminer-Samuel B. Rothberg Attorney-John R. Faulkner and Glenn S.Arendsen [5 7] ABSTRACT A monolithic seal member made of zinc oxide ispositioned in the housing of a gas turbine engine where the seal memberrubs against a rotating ceramic regenerator. The seal member hasexcellent friction and thermal shock characteristics that make ituniquely suited for use in the gas turbine engine. Adding small amountsof calcium fluoride to the seal member reduces the coefficient offriction without changing significantly the thermal shockcharacteristics. The zinc oxide also can be applied as a coating to aceramic substrate.

6 Claims, 1 Drawing Figure PATENTEDMAR 7 I972 I 3 647, 2 2 8 INVENTOROO/V/VZO J. 16346670)" ATTORNEYS CERAMIC REGENERATOR SEALS SUMMARY OFTHE INVENTION This application relates to the subject matter of US. Pat.applications Ser. No. 854,397, filed Sept. 2, 1969 and Ser. No. 872,328,filed Oct. 29, 1969, now U.S. Pat. No. 3,601,414.

In addition to friction and wear properties, seals for the ceramicregenerator of a gas turbine engine must be able to withstandconsiderable temperature gradients during steadystate operation andrapid temperature increases during engine startup. Such temperatureincreases or thermal shocks arise from the fact that engine temperaturescan increase from am bients of below zero to approximately l,800 F.within about 3 seconds. Seals made by coating metallic substrates withfriction and wear reducing compositions are fairly satisfactory, but therepeated thermal shocks and temperature gradients tend to separate thecoatings from the substrate.

Continuing efforts to improve the life of regenerator seals resulted inthe exclusively ceramic seal member of this invention. The seal membershave an exceptionally good combination of wear, friction and thermalproperties. A monolithic seal member consisting essentially ofstructurally integral zinc oxide can be produced inexpensively and hasthermal properties that include excellent thennal shock resistance andability to retain dimensional shape under significant temperaturegradients. Wear and friction properties are improved by adding smallamounts of ceramic halogenic salts. Such zinc oxide materials also canbe applied as coatings to substrates made of other ceramic materials.

The seal members are located nonrotatably in the housing of the gasturbine engine where the rotating ceramic regenerator slides againstsurfaces thereof. An isostatic pressing process preferably is used tomanufacture the monolithic seal members. Powdered zinc oxide having aninitial particle size of about one micron is mixed with any additionalingredients and prereacted in air at 2,500 F. for about 2 to 3 hours.The resulting material is crushed and ball milled to a powder having auniform particle size of about 4 microns. 'A binder is added to thepowder and the mixture is granulated and isostatically pressed into theshape of the monolithic seal member. After pressing, the seal member isheated to about 600 F. to burn out the binder and then is sintered atabout 2,000 -2,600 F. to form a seal member having a density of 60 to100 percent of theoretical. Monolithic seal members having a densitywithin the range of 7595 percent of theoretical have the bestcombination of strength and wear properties.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is an exploded view of theregenerator portion of a gas turbine engine showing the structure andlocation of several monolithic seal members made according to thisinvention.

DETAILED DESCRIPTION Referring to the drawing, the regenerator portionof a gas turbine housing contains a diagonal wall 12 that divides itsinterior into two semicircular gas flow passages 14 and 16. A cross armseal 18 fits nonrotatably on top of wall 12 and two C seals 20 and 22surround the outer peripheries of passages 14 and 16 respectively.

A disc-shaped ceramic regenerator 24 is mounted rotatably on top ofseals 18, 20 and 22. A D-shaped seal 26 rests on top of regenerator 24and is held in place by a cap member (not shown).

Disc 24 is rotated by accessory drive components of the engine in theconventional manner. Air from the engine compressor passes through thesegment of disc 24 that subtends passage 16 and through passage 16 tothe engine combustion chamber. Hot exhaust gases from the turbine wheelor wheels pass through passage 14, the segment of disc 24 above passage14 and the space defined by D seal 26 into an exhaust passage defined bythe cap. Disc rotation transfers heat from the hot exhaustgases to thecooler air.

Each of seal members 18, 20 and 22 is an exclusively ceramic memberconsisting essentially of structurally integral zinc oxide. Seal member26 can be made of the same material but it generally does not encountertemperatures necessitating the excellent thermal properties. Sealmembers made entirely of zinc oxide exhibit a coefficient of friction ofless than about 0.5 when rubbing against a ceramic regenerator attemperatures between l,200 and l,800 F. The thermal shock properties andthe ability to withstand temperature gradients of the seals greatlyexceeds previous coated metal seal members.

Adding up to 40 percent of a halogenic salt of calcium, potassium,magnesium, cerium, barium and strontium reduces the coefficient offriction of the seal members without significantly affecting the thermalshock properties. About 20-30 percent of the halogenic salt provides thebest friction reductions. Particularly useful salts are calcium fluorideand potassium chloride, which provide the best combination of friction,wear and strength.

A powder pressing operation can be used to manufacture the monolithicseals of this invention. Powdered zinc oxide is mixed with anyadditional ingredients and prereacted at a temperature of about 2,500"F. to reduce thermal shrinkage. The reaction product is crushed to about20 mesh in a hammer mill and then ball milled for about 16 hours oruntil the product reaches a relatively uniform particle size of about 4microns. A wax emulsion binder that consists of about 40 percent byweight of wax in water is added to the powder and the mixture isgranulated in a blender. Granulation assists in reducing compactionduring the actual pressing operation.

The granulated material is placed in an isostatic or dry press andpressed into the desired shape using pressures of about 10,000 p.s.i.After pressing, the seal member is heated to burn out the binder andthen is sintered at a temperature of about 2,000-2,600 F. to produce astructural integral monolithic seal member. The seal member then isground to final dimensions if necessary. Surface finishing techniquesgenerally are not required but can be used to improve initial matingcharacteristics.

Seals intended for use at a relatively low temperature preferably aresintered in the higher portion of this range while seals intended foruse at higher temperatures of about 1,800 F. can be sintered anywherewithin the range. Sintering temperature and operating temperatureapparently combine to produce final seal compositions having the desiredfrictional properties.

Useful seals also can be produced by flame spraying or painting the zincoxide material directly onto other ceramic materials such as aluminumoxide, silicon nitride, silicon carbide, lithium aluminum silicate, ormullite (aluminum silicate). Such other ceramic materials serve assubstrates for the coating of the zinc oxide material to provide anexclusively ceramic seal member having excellent thermal properties.

In an alternate slip casting process, water is added to the product ofthe hammer mill and the mixture is ball milled under wet conditions toan extremely fine particle size. The resulting mixture is slip cast intothe desired seal shape and dried by heating the resulting member to atemperature of about Sintering then is carried out in the same manner asthe dry pressing operation and can be followed by grinding the resultingproduct to final shape.

Thus this invention provides structurally integral seal members madeexclusively of ceramic materials for gas turbine regenerators. The sealmembers can be monolithic or can comprise a zinc oxide coating on otherceramic substrates.

I claim:

1. ln a gas turbine engine having a ceramic regenerator mounted forrotation relative to an engine housing, a sliding seal for saidregenerator comprising an exclusively ceramic seal member consistingessentially of structurally integral zinc oxide, said member beingnonrotatably located in said housing to slide against said regeneratorwhen said engine is operating.

2. The gas turbine engine of claim 1 in which the seal member containsup to about 40 weight percent of a halogenic salt intimately dispersedthroughout said zinc oxide.

3. The gas turbine engine of claim 2 in which the halogenic salt iscalcium fluoride or potassium chloride.

4. The gas turbine engine of claim 3 in which the seal member ismonolithic and has a density of 75 to 95 percent of theoretical density.

5. The gas turbine engine of claim 1 comprising a ceramic substrate forsaid zinc oxide of aluminum oxide, silicon

2. The gas turbine engine of claim 1 in which the seal member containsup to about 40 weight percent of a halogenic salt intimately dispersedthroughout said zinc oxide.
 3. The gas turbine engine of claim 2 inwhich the halogenic salt is calcium fluoride or potassium chloride. 4.The gas turbine engine of claim 3 in which the seal member is monolithicand has a density of 75 to 95 percent of theoretical density.
 5. The gasturbine engine of claim 1 comprising a ceramic substrate for said zincoxide of aluminum oxide, silicon nitride, silicon carbide, lithiumaluminum silicate, or aluminum silicate.
 6. The gas turbine engine ofclaim 1 in which the seal member is a monolithic member made fromsintered zinc oxide, said monolithic member having a density of at least60 percent of theoretical.